Band-pass input circuit



June 13, 1950 E. J. H. BUSSARD BAND-PASS INPUT CIRCUIT 2 Sheets-$heet 1Filed Jan. 3, 1949 INVENTOR.

fM/VEE) J 5055420 My; 7%; %4 m Arm s June 13, 1950 E. J. H. BussARDBAND-PASS INPUT CIRCUIT Filed Jan. 5, 1949 2 Sheets-Sheet 2 GAINDECIBELS I200 I400 FREQUENCY- KILOCYCLES INVEN TOR. [MMEEI 7-K 51/554213 Patented June 13, 1950 UNITED STATES PATENT OFFICE BAND-PASS INPUTCIRCUIT Emmery J. H. Bussard, Cincinnati, Ohio, assignor to AvcoManufacturing Corporation, Cincinnati, Ohio, a corporation of DelawareApplication January 3, 1949, Serial No. 68,771

1 Claim.

The present invention relates to antenna input circuits for radioreceivers.

The primary object of the invention is to provide a radio frequencyinput circuit which requires no manual tuning operations but whichcovers a wide frequency band, such as the AM broadcast band. (540kc.-l600 kc.) with high voltage gain for desired signals andgreat-attenuation of undesired signals outside the broadcast band.

Another object of the invention is to provide a fixed-tuned radiofrequency input circuit which has substantially uniform transferconstant and voltage step-up characteristics over a 'wide frequencyband, and which at the same time provides desired selectivity anddiscrimination against image signals and signals of intermediatefrequency.

A further object of the invention is to provide an antenna input circuitcomprisingan autotransformer, in which a part of the transformer notincluded in the primary is employed in conjunction with a shuntcapacitor as an anti-resonant wave trap to discriminate againstundesired signals.

For a better understanding of the invention, together with other andfurther objects, advantages, and capabilities thereof, reference is madeto the following detailed description of the accompanying drawings, inwhich there are disclosed two illustrative R. F. antenna input circuitsin accordance with the invention.

In the drawings:

Fig. l is a circuit diagram of a preferred form of radio frequencyantenna input circuit in accordance with the invention;

Fig. 2 is a circuit diagram of a modified form in which the antenna loopis employed as the primary;

Fig. 3 is a performance curve showing the response characteristic of theFig. 1 embodiment; and Fig. 4 is a perspective View of a transformersuitable for incorporation in the Fig. 1 embodiment.

As is well known to those skilled in the art, the radio frequencysection of a receiver includes the coupling from the antenna to the gridof the first tube, such coupling, being generally referred to as theantenna input circuit. The chief purposes of this circuit are toprovideselectivity against image and intermediate frequency and otherinterfering signals and at the same time to afford voltage gain betweenthe antenna and the grid-cathode input circuit of the first R. F.amplifier tube. It is customary to provide a tuned radio frequency stagebetween the antenna input circuit and the mixer or frequency converter,or a tuned antenna input circuit coupled to the converter. Capacitors orinductors in both the antenna input circuit and the oscillator tankcircuit areadjusted to select the desired signal channel. However, whenpush-button tuning or equivalent automatic selector arrangements areemployed, it is desirable to omit the adjustments of the-antenna inputcircuit and to provide a band pass antenna input circuit which passes sinals within the entire band under consideration, such as the AMbroadcast band, with substantially uniform voltage step-upcharacteristics over the entire band, and also with the requiredselectivity and discrimination performance characteristics.

To provide such a, circuit has for a long time posed a very difiicultproblem, and the need for a simplified push-button tuning system hasprompted many endeavors to solve it.

Referring now specifically to Fig. 1 of the drawings, there is shown alow impedance loop antenna II and a converter tube Hi. The purposetowhich the present invention is directed is to couple the loop to thecontrol electrode of converter tube ill in such a way as to eliminatethe conventional antenna input variable tuning and at the same time tomeet the performance requirements mentioned above. The performanceequivalent of an over-coup1ed double-tuned transformer has been providedby the invention. Loop I I is shunt-connected to primary I3 ofautotransformer l6, and this section of the input systein is resonatedto about the mid-frequency range by means of fixed capacitor 52.Specifically, the loop I i and its series tuning capacitor I2 arecoupled to the primary [3 of an auto-transformer I6. The primary I3 istuned approximately to the center of the AM broadcast band, for example,by capacitor l2 in series with low impedance loop I l and by thedistributed capacitance ll of the loop and primary. The secondary l3,l4, l5 of transformer i6 is tuned by shunt capacitor l8 approximately tothe center frequency of the AM standard broadcast band. There are shownin association with the circuit the usual coupling capacitor [9 and agrid resistor 28 leading to source of AVG voltage (not shown). Theelements I9, 29 couple the antenna input circuit to the controlelectrode of converter I0. Local oscillator voltage is injected inconventional manner through a capacitor 25 coupled to the output of anoscillator (not shown). The low voltage terminals of the loop, coil I3,capacitor I8, resistor 22, and the cathode of tube 10 are grounded at23.

Portions i4 and I5 of the transformer secondary, together with shuntcapacitor 24, comprise a trap circuit which is anti-resonant at the I.F. frequency. In a particular embodiment of the invention which wassuccessfully reduced to practice, the receiver was of the doublesuperheterodyne type and the first I. F. frequency was 5825 kilocycles.The trap circuit comprising secondary portions [4, l5 and capacitor 24provides very effective discrimination against unwanted.

signals of I. F. frequencies. The other parameters employed in thisillustrative embodiment were as follows:

Inductance of loop ll micro-henries 15.65 Capacitor l2micro-micro-farads '750 Distributed capacitance of coil l3 do 11Capacitor l8 do 220 Inductance of coil [3 micro-hem'ies 30 Sum ofinductance of coils l3, l4, and

it: in series and twice the mutual inductance between the primary andsecondary micro-henries 215 Mutual inductance between primary l3 andsecondary i3, l4, l5 micro-henries 38 Shunt tuning of loop H ki1ocycles2550 Distributed capacitance of loop ll micro-micro-farads 63 Resistor22 ohms 5600 Tube l0 Type SAC? Resistor 23 megohm 1 Capacitor 2|micro-micro-farads 10 The gain and selectivity of. the system can bematerially improved by resonating the loop near to the high end of thedesired range. 2550 kilocycles proved satisfactory in the example cited.That is, loop H and shunt capacitance I! are preferably resonant atabout 2550 kilocycles. Loop H and capacitor I2 and primary 13 areresonant at the center of the broadcast band. Resistor 22 is employed tolimit the peak gains in such a manner that the gain ratio of peaks tovalley is not excessive.

Referring now specifically to Fig. 3 of the drawings, the performancecurve there shown illustrates the band pass input gain characteristic ofa receiver having an antenna input circuit in accordance with theinvention and employing the illustrative parameters above mentioned, ascompared to a like receiver employing a typical manually tuned inputcircuit. The response curve is plotted on a. system of Cartesiancoordinates with frequency in kilocycles as abscissae and gain indecibels as ordinates. It will be observed that the responsecharacteristic is indicative of a double-tuned over-coupled system, onehump occurring at about 660 kilocycles and the other at about 1250kilocycles. This gain characteristic curve indicates a uniformity whichis commercially acceptable throughout the standard broadcast band. Suchresults cannot be achieved by conventional methods of coupling. Acoupling coefficient on the order of 0.55 is. re-

quired for coverage of the AM broadcast band. for example. I 4

4 Sensitivity measurements of a receiver with these illustrativeparameters give the following results:

Frequency in Kilocycles Input Output Micro- In this illustrativereceiver the band pass input transformer, as shown in Fig. 4, was woundwith 15-strand No. 44 Litz wire in three coils l3, i4, 15, each /64 inchin width, spaced about inch, on a form 25 having a inch diameter. Thesecoils are identical, each having 35 turns.

Referring now specifically to Fig. 2 of the drawings, there is shown anembodiment in which loop 28 is common to both input and output of thecoupling network. This coupling arrangement includes loop 28 and seriesinductors 29 and 30 as, the secondary. Loop 28 is the primary of aneffectively over-coupled double-tuned system. Like reference numeralsare employed to designate elements which are the same as those shown inthe Fig. 1 embodiment. In the Fig. 2 embodiment the secondary portionsti] and 29 are shunttuned by their own distributed capacitance toprovide the anti-resonant trap circuit for I. F. frequency rejection.The loop is tuned by a shunt capacitor 3|. The primary is tuned bycapacitor 31, and the secondary by capacitor l8, each to the approximatemid-frequency of the band to be received.

The auto-transformer feature provides the required coupling for peakdisplacement and for uniform voltage stepup. The invention is ofparticular utility in a double superheterodyne receiver in which thefirst intermediate frequency is substantially higher than the band to bereceived. Its use permits the antenna tuning condenser to be omitted,avoids tracking problems, and permits complete range tuning with a verysmall variable capacitor.

While there have been shown and described what are at present consideredto be the preferred embodiments of the present invention, it will beunderstood by those skilled in the art that various substitutions ofequivalents may be made therein without departing from the true scope ofthe invention as defined by the appended claim.

Having fully described and disclosed my invention, I claim:

In a radio receiver, an antenna input circuit adapted to afford arelatively uniform response throughout the AM broadcast band withoutmanual tuning, comprising a vacuum tube having an input and an outputcircuit, an input network including a low impedance loop, a high-Qautotransformer having primary and secondary windings, a first capacitorcoupling said low impedance loop across the primary of saidautotransformer so as to form a series resonant network tuned to themid-frequency of said band, a second capacitor means connected in shuntacross that part of the secondary of said autotransformer which is notcommon to the primary to form a rejector network anti-resonant at thereceiver intermediate frequency, a third capacitive means coupled acrossthe end terminals of said autotransformer to series resonate at themid-frequency of said band the loop circuit in which the third capacitormeans is included, and means coupling the input circuit of said vacuumtube across said third capacitive means, whereby a large gain betweenthe low impedance loop and the input of said vacuum tube and a wide bandrelatively uniform response characteristic is realized, and wherebyundesired signals of intermediate frequency are rejected by saidanti-resonant rejector circuit.

EMMERY J. H. BUSSARD.

REFERENCES CITED The following references are of record in the file :ofthis patent:

UNITED STATES PATENTS

